29 CFR §2560.503-1 (g)(1)

(g) Manner and content of notification of benefit determination.

(1) Except as provided in paragraph (g)(2) of this section, the plan administrator shall provide a claimant with written or electronic notification of any adverse benefit determination. Any electronic notification shall comply with the standards imposed by 29 CFR 2520.104b-1(c)(1)(i), (iii), and (iv). The notification shall set forth, in a manner calculated to be understood by the claimant –

(i) The specific reason or reasons for the adverse determination;

(ii) Reference to the specific plan provisions on which the determination is based;

(iii) A description of any additional material or information necessary for the claimant to perfect the claim and an explanation of why such material or information is necessary;

(iv) A description of the plan’s review procedures and the time limits applicable to such procedures, including a statement of the claimant’s right to bring a civil action under section 502(a) of the Act following an adverse benefit determination on review;